Water is generally used as a medium in traditional dyeing processes using reactive dyes for textiles. In addition to a waste problem caused by the hydrolysis of reactive dyes, a high water consumption and the discharge of highly polluted wastewater containing dyes and various auxiliaries also cause an increasingly serious impact on the environment. Particularly in recent years, with an increasing emphasis on environmental issues, workers in the textile industry are also increasingly committed to the development of new dyeing and finishing processes that are environmentally friendly and water-saving. In this context, solvent assisted dyeing processes using reactive dyes have been further developed.
The topic of using an organic solvent instead of water as a textile dyeing and finishing medium has been studied by textile workers for several decades. The easy separation and recovery characteristics of organic solvents can effectively reduce the discharge of polluted wastewater. Particularly in recent years, some documents have reported replacing water with a low toxicity or non-toxic organic solvent as a main dyeing medium for reactive dyes, which can greatly reduce the water consumption in the dyeing process while resulting in a dyeing depth that is similar to that of traditional water bath dyeing; moreover, the dyeing can be done at a lower temperature without the addition of any salt for accelerating dyeing; and after the dyeing is complete, most of the organic solvent is recycled and it will not be directly discharged to the environment. It can be seen that this is a very energy-saving and environmentally friendly dyeing process.
The main principle of this process for dyeing with a reactive dye using an organic solvent that replaces part of the water medium (i.e., a solvent assisted dyeing method) is based on the use of the reverse micelle theory. Reverse micelles are nanoscale spherical aggregates that are self-assembled from water, an oil and a surfactant at a specific ratio under certain conditions. There is a stable aqueous microenvironment, i.e., so-called water-pool, in the interior region of the micelle. Under certain conditions, some hydrophilic substances such as enzymes can be solubilized in the water-pool without losing their activities. Likewise, hydrophilic dyes, such as reactive dyes, can also be solubilized in the water-pool of the reverse micelle to form a reverse micelle-reactive dye dyeing system with an organic solvent as the main medium. In this dyeing system, textiles can achieve similar dyeing effects to the effect of an all-water bath dyeing system.
However, due to the dyeing mechanism, the reactive dye often undergoes a fixation reaction under basic conditions so as to achieve a good colour fastness. In some documents regarding studies on solvent assisted dyeing processes using reactive dyes, a process involving pre-padding with an aqueous sodium carbonate solution and then placing a sodium carbonate-containing textile into a reverse micelle-reactive dye dyeing system for dyeing is described. Although in this method two steps of dyeing and fixing can be performed in one step, the problems of reduced levelness, reduced fixation rate, dye wasting, etc., can still be caused due to the hydrolysis of the reactive dye in a basic aqueous environment.
In other studies, textiles are pretreated with cationic modifiers, followed by solvent assisted dyeing using reactive dyes. This method can significantly improve the textile dyeing performance and increase the utilization of the dyes. However, since the cationic modifiers are used for treating the textiles, water is still required as a medium, leading to the discharge of a large amount of waste water; some cationic modifiers have higher costs and may have adverse effects on the environment; if the cation modification treatment process is improper, and the modifier does not penetrate into the interior of fibres, a phenomenon of ring dyeing of the textile may also be caused, affecting the colour brightness and colour fastness of the textile.
Therefore, under the premise of ensuring a dyeing depth, uniformity and colour fastness, minimizing the waste of hydrolysis of the reactive dye in the solvent-assisted dyeing process and to avoid a special pretreatment that is water consumptive and has a potential negative impact on the environment has become a problem that needs to be solved in solvent assisted dyeing processes.